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Experiment with compensating balance resistor

Posted by cellai@space.it on July 9, 2008 at 06:43:01:
I have provisionally tried a 5.4 Ohm (in series with the cathodes of the SEPP lower triodes) for fixed compensation of the SEPP nominal gain imbalance (0% nominal gain imbalance @8 Ohm).

This simple modification was mainly aimed at assessing if I was able to listen to some audible difference (with and without such a fixed balance compensation).

The limited listening test performed so far has been clearly based on impressions 'prior and after' the modification (hence not by a live comparison of the switched outputs of two simultaneously available amplifiers), as the OTL amplifier HW had to be always the same.

Theoretically the calculated 2nd harmonic (closed loop) distortion should have gone down from a supposed level of something less than -30 dB (no compensating resistor) to something better than -40 dB (with compensating resistor).

I must say that if there is a difference (and I believe there must be) it can be only ascertained by some direct measurement and spectral analisys (not available at this time).

By my ears only (over some hours of listening to my favourite classic music) I can neither say if there is a significant difference nor if the sound is subjectively better (sorry, probably I am not a 'golden ear').

This is the reason why I am not going to modify further the present OTL design.

A fully automatic SEPP gain imbalance compensation will be the matter of my next OTL (likely a true 'modified Futterman' like yours).

For sake of completeness the calculated compensating resistor for 16 Ohm speakers is 10.65 Ohm.

A spreadsheet is available to potentially interested DIY readers for calculation of compensating resistor (send a request to cellai@space.it).

Best Regards
Luca